The present invention relates to an electromagnetic air control valve for an internal combustion engine fuel supply device, in particular, for controlling air supply along a duct connecting two zones, respectively up- and downstream from the throttle valve.
Valves of the aforementioned type substantially comprise a body having a cylindrical sliding seat, and an opening enabling the passage of air and coming out inside said seat; a plunger sliding axially inside said seat, for regulating said air supply opening; and an electromagnet having a core designed to control displacement of the plunger when the electromagnet is energized.
Said core presents a rod, to the end of which the plunger is fitted by means of a threaded nut screwed on to the threaded end of the rod and resting on an annular collar on the same. Between the rod and a shoulder on the core, a helical spring is inserted coaxial with the rod.
Said plunger substantially comprises an outer wall of nonmagnetic material, designed to mate with the surface of said sliding seat; and an inner bush of magnetic material, connected to said wall by means of adhesive.
Valves of the aforementioned type present a number of drawbacks.
First and foremost, no strictly accurate relationship can be determined between displacement of the plunger, and consequently the air supply opening through the valve, and the amount of current supplied to the electromagnet; which relationship depends on the rigidity of the spring between the core and plunger, and on preloading of the spring at the valve assembly stage. As the springs used on different valves, however, cover a fairly wide rigidity range, and preloading of the spring at the assembly stage cannot be regulated by calibrating the valve, actual performance of the air supply opening through the valve as a function of current supply to the electromagnet may differ widely in relation to theoretical performance.
Moreover, axial displacement of the plunger inside the cylindrical sliding seat may be accompanied by friction of such an extent as to further affect the relationship between the air supply opening and current supply to the electromagnet. For part of its stroke, in fact, the plunger comes partially out of the seat, for ensuring the electromagnetic force exerted by the core on the plunger is maintained constant throughout the entire stroke. As such, the guiding action performed by the seat on the lateral surface of the plunger is not constant throughout the entire stroke of the plunger, and may even be totally insufficient when part of the plunger is actually outside the seat.
Moreover, impurities in the air fed through the valve are invariably deposited between the outer surface of the plunger and the sliding seat, thus further increasing friction between the two parts, which may even result in seizing of the plunger itself.
Finally, fatigue on the plunger may result in the bush coming away from the wall of the same.